Spontaneous formation of OH radical and H2O2 at the liquid-ice interface

Recently, intensive new particle formation (NPF) events have been observed in the upper troposphere/lower stratosphere (UTLS), where ice formation is predominant. Atmospheric oxidants including hydroxyl radical (OH∙) and hydrogen peroxide (H₂O₂) play important roles in these NPF events. However, the underlying formation mechanisms of OH∙ and H₂O₂ remain poorly understood. Here we propose that spontaneous formation of OH∙ and H₂O₂ is occurring at the liquid-ice interface during ice freezing, acting as so far unconsidered source of oxidants in the UTLS. This production is induced by the Workman-Reynold effect which predicts that a freezing potential appears in a freezing salt solution and thus an electric field is formed at the liquid-ice interface.

In this work, solutions containing disodium terephthalate (TA, ~5 x 10^-5 M) were frozen either by immersion into an ethanol bath (-20 ºC) or into liquid nitrogen, and then melted. These steps were repeated creating freezing-melting cycles (n = 0-25). The solutions were then analyzed by a fluorescent spectroscopy to monitor the formation of 2-hydroxyterephthalic acid (TAOH), a product of the reaction of TA with OH∙. The production of TAOH was observed to be positively correlated with the number of freezing-melting cycles, demonstrating the formation of OH∙ during the freezing process. A series of salt solutions containing either NaCl, NH₄Cl, NaBr, NaI, NaIO₃ at different concentrations i.e.,10^-6-10⁰ M were also frozen and melted, and analyzed for their content in H₂O₂. Also here, our results confirmed the H₂O₂ production at the liquid-ice interface for the freezing salt solutions. In the case of NaCl, the maximum H₂O₂ production was observed at the concentration of ~10^-4 M. Furthermore, the production rate of H₂O₂ at the NaCl concentration range of 10^-4-10^-2 M, was in agreement with the known Workman-Reynold freezing potential values. In order to investigate the role of OH∙ recombination in the H₂O₂ formation, mixed solutions of NaCl (~10^-4 M) and TA (~5 x 10^-5 M) subjected to different freezing-melting cycles were analyzed. The production rate of H₂O₂ was higher than that of TAOH by a factor of ~65, suggesting less significant effect of TA as a OH∙ scavenger on H₂O₂ formation. Overall, our experimental results provide direct evidence that OH∙ and H₂O₂ are formed spontaneously at the liquid-ice interface due to the Workman-Reynold effect. This study could improve our ability to describe the multiphase oxidation processes of the UTLS regions.